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8.4: Carbocation rearrangements

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    225805
  • Carbocation rearrangements are common in organic chemistry and are defined as the movement of a carbocation from an unstable state to a more stable state through the use of various structural reorganizational “shifts” within the molecule.

    Introduction

    Whenever an alkyl halide, alcohol or alkene is transformed into a carbocation, the carbocation may be subject to rearrangement. There are two types of carbocation rearrangements: a hydride shift and an alkyl shift.  Once rearranged, the resultant carbocation will react further to form a final product which has a different alkyl skeleton than the starting material.  In practice, we will often see a mixture of isomeric products formed from both the unrearranged and the rearranged carbocations.  We will examine in detail how this occurs in SN1 substitution reactions starting with alcohols.  This is related to the SN1 substitution reactions with alkyl halides that you have seen previously in section 8.3., but in this case the OH is made into a leaving group via an acid-base reaction (so it leaves as water).  We will study this reaction further in chapter 9.

    Hydride shift

    We see that the formed carbocation can undergo a rearrangement called a hydride shift. This means that a hydrogen moves over from one carbon to a neighboring (less substituted) carbon. We often see a hydride shift in the reaction of an alcohol with HBr, HCl, and HI.  Below is an example of a reaction between an alcohol and hydrogen chloride:

    Slide2 (2).jpg

    GREEN (Cl) = nucleophile     BLUE (OH) = leaving group     ORANGE (H) = hydride shift hydrogen  RED(H) = remaining hydrogen

    The alcohol portion (-OH) has been substituted with the Cl atom. However, it is not a direct substitution of the OH.  The mechanism is as follows:

    Alkyl shift

    Not all carbocations have suitable hydrogen atoms (either secondary or tertiary) that are on adjacent carbon atoms available for rearrangement. In this case, the reaction can undergo a different mode of rearrangement known as an alkyl shift (or alkyl group migration). The alkyl shift acts in a similar way to the hydride shift.  The shifting alkyl group carries its electron pair with it to form a bond to the neighboring carbocation. The shifting alkyl group and the positive charge of the carbocation switch positions on the molecule.  Tertiary carbocations are much more stable than primary or secondary carbocations, so we see often an alkyl shift on a primary or secondary carbocation to form a tertiary carbocation.  For example:

    Slide7.jpg

    Carbocation rearrangements in other types of reaction

    As mentioned above, any reaction involving a carbocation intermediate may be subject to rearrangement.  E1 elimination reactions, which will be covered in the next section, can also include a hydride or alkyl shift, leading to a more substitued alkene. Also carbocation rearrangements may occur during certain electrophilic additions, to be covered in chapter 10.

    References

    1. Vogel, Pierre. Carbocation Chemistry. Amsterdam: Elsevier Science Publishers B.V., 1985.
    2. Olah, George A. and Prakash, G.K. Surya. Carbocation Chemistry. New Jersey: John Wiley & Sons, Inc., 2004.
    3. Vollhardt, K. Peter C. and Schore, Neil E. Organic Chemistry: Structure and Function. New York: Bleyer, Brennan, 2007.

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